1. Field of the Invention
This invention relates to lithography and more specifically to charged particle (e.g. electron beam) lithography using microcolumns to direct write images onto a wafer.
2. Description of the Prior Art
Most lithography now used for fabricating semiconductor devices uses light which passes through a mask, where the mask carries the image to be transferred to a photosensitive resist layer on a substrate. A second type of well known lithography is electron beam lithography. Instead of using light, this uses an electron beam (but no mask) to form an image on a substrate. The substrate is coated with a layer of resist sensitive to the incident electron beam. The resist in either case is then developed and the exposed areas then either remain or are removed, defining a pattern on a surface of the wafer. Subsequent steps etch away the exposed portions of the wafer surface to define semiconductor features.
To date, electron beam lithography has been used mostly to fabricate the masks rather than the semiconductor wafers. The masks themselves are then used in the photolithography, as disclosed above. However, it is also known to use electron beam lithography to direct write features onto a semiconductor wafer. Typically this is only used for low throughput systems where a small number of semiconductor chips are needed, since the direct write approach is relatively slow. Of course it has the advantage of eliminating the masks and also providing very small feature sizes (better resolution) due to the nature of the electron beams compared to the much longer wavelength light used in photolithography. Thus to date practical applications of high resolution electron beam lithography are typically limited to mask making and manufacturing of highly specialized integrated circuits, due to the low throughput and high equipment cost for electron beam lithography.
However, since the general trend in semiconductor fabrication is to reduce minimum feature size and there is an expectation of a minimum feature size below 100 nanometers in the next 10 years, photolithography is becoming increasingly expensive and may not offer sufficient resolving power. Minimum feature size is typically the minimum width of a portion of a transistor as defined the lithography process which in turn defines the overall size of the transistor and hence the number of transistors or other semiconductor devices which may be provided on a single integrated circuit.
While integrated circuits are typically called chips, at the wafer level and prior to packaging they are often referred to as die. That terminology is used herein to refer to a single semiconductor substrate which will later become a semiconductor integrated circuit. A typical semiconductor wafer contains many (for instance hundreds or thousands of) such die arranged in a grid.
Therefore while it has been widely recognized that electron beam lithography has possibilities for direct writing of mass production semiconductor wafers, so far this has not been commercially feasible.
Also known in the electron beam lithography field is a technology called microcolumns. A typical electron beam lithography machine has a single source of electrons, an associated accelerator (electrostatic) device for accelerating the electrons, and a set of elements which are typically coaxial electro-magnets for focusing the beam onto the substrate. However, it is known (see e.g. U.S. Pat. Nos. 5,155,412 and 5,122,663 to IBM and "Electron-beam microcolumns for lithography and related applications", incorporated herein by reference) to provide an array of so called microcolumns wherein each individual microcolumn is a complete electron beam column including an electron beam source, an accelerator or extractor electrode, a deflector electrode for scanning the beam, and a electrostatic lens for focusing the beam. Such microcolumns have a typical diameter of approximately 1 to 2 centimeters. A two dimensional array of such microcolumns has been proposed for lithography.
Also, while the disclosure herein is mostly directed to electron beam lithography, electrons of course are just one type of charge particles. Also known are ion beam sources which instead of emitting electrons emit other charged particles. Unlike an electron beam system, instead of merely requiring a source of electric current to the source, a source of atoms (a gas) must be provided. Hence such devices are generally slower in writing speed and more complicated than electron beam devices and so far have not been used commercially for lithography.
In any case, it would be desirable to provide a method of increasing throughput (production rates) in semiconductor fabrication using electron or ion beam technology so as to allow direct writing of semiconductor wafers for mass production of high volume integrated circuits. So far, this has not been feasible.